Thick film dielectric electroluminescent devices as exemplified by Applicant's U.S. Pat. No. 5,432,015 exhibit superior characteristics to that of traditional TFEL displays. High performance red, green and blue phosphor materials have been developed for use with thick film dielectric structures to provide increased luminance performance. These phosphor materials include europium activated barium thioaluminate based materials for blue emission, terbium activated zinc sulfide, manganese activated magnesium zinc sulfide or europium activated calcium thioaluminate based materials for green emission, as well as traditional manganese activated zinc sulfide that can be appropriately filtered for red emission.
The thin film phosphor materials used for red, green and blue sub-pixels must be patterned using photolithographic techniques employing solvent solutions for high resolution displays. Traces of these solutions remaining in the display following photolithographic processing together with reaction of moisture or oxygen present in the processing environment may react chemically with certain phosphor materials that are sensitive to oxidation or hydrolysis reactions to cause performance degradation of the completed display. Continued chemical reactions during operation of the display may cause continued performance degradation thereby shortening the life of the display.
To overcome such performance degradation problems, researchers have proposed the use of various silicon materials including silicon nitride (Si3N4), silicon oxide (SiO2) and silicon oxynitride (SiON) as insulators in conjunction with phosphor materials to help decrease degradation of the phosphor. Insulator or barrier layers of these materials are traditionally taught for use with zinc sulfide phosphors in TFEL, OLED and EL devices as exemplified in U.S. Pat. Nos. 4,188,565, 4,721,631, 4,774,435, 4,880,661, 4,897,319, 4,954,747, 5,598,059, 5,644,190, 6,322,860, 6,388,378 and 6,392,334 as well as by U.S. Patent Application Nos. 2001/0055458, 2002/0001733, 2002/0005506, 2002/0006051, 2002/0037430 and 2002/0084464 and in Mikami et al., (Proceedings of the 6th International Conference on the Science and Technology of Display Phosphors (2000) pp 61–64) and J. Ohwaki et al., (Review of the Electrical Communications laboratories Vol. 35, 1987).
Silicon materials have also been suggested for use as a film insulating layer on top of a barium magnesium oxythioaluminate phosphor used within EL panels as for example disclosed in U.S. Patent Application No. 2002/0031685.
The Applicant's U.S. Patent Application No. 2002/0094451 teaches that silicon oxynitride is not desirable for use as an insertion layer with an europium activated barium thioaluminate phosphor but rather that barium titanate is suitable as it provides for superior luminance and increased barrier properties for the diffusion of contaminant species such as lead from the thick dielectric layer into the phosphor. Therefore one skilled in the art would be discouraged to use silicon oxynitride (SiON) with thioaluminate phosphors based on the teachings of the prior art.
While the aforementioned references and patents may teach the use of a conventional silicon nitride and silicon oxynitride as “barrier” or “insulator” material for the purpose of preventing reaction of a zinc sulfide phosphor with water from the ambient environment or some other “stabilizing” type function, there remains a need to provide an improved rare earth activated alkaline earth thioaluminate phosphors used in thick dielectric film electroluminescent displays in order to provide both improved luminance and a long operating life with minimal degradation.